PastPaperHero | Relevancy and reasons for excluding relevant evidence - Proper subject matter for expert testimony

Learning Outcomes

This article explains relevancy and the exclusion of relevant evidence in the context of lay and expert testimony, including:

applying Rules 401–403 to determine relevance and discretionary exclusion in common MBE scenarios;
distinguishing lay opinion (Rule 701) from expert opinion (Rule 702) and spotting misclassified testimony;
identifying when a proposition is proper subject matter for expert testimony and when it must be presented as lay evidence or excluded altogether;
using the Daubert/Kumho reliability framework and the trial judge’s gatekeeping role to assess scientific, technical, and specialized evidence;
recognizing expert opinions that improperly give legal conclusions, address a criminal defendant’s mental state under Rule 704(b), or vouch for witness credibility;
evaluating the permissible bases of expert opinions under Rules 703–705 and when underlying inadmissible data may reach the jury;
deploying Rule 403 strategically to challenge marginally helpful but confusing, cumulative, or unfairly prejudicial opinion evidence;
analyzing MBE-style fact patterns that integrate relevancy, 403 balancing, and the limits on lay and expert testimony.

MBE Syllabus

For the MBE, you are required to understand relevancy and exclusion of evidence, and the rules governing opinion testimony, with a focus on the following syllabus points:

Definition of relevance under Rules 401–402 and the general rule of admissibility of relevant evidence.
Discretionary exclusion of relevant evidence under Rule 403 (unfair prejudice, confusion, misleading the jury, undue delay, waste of time, cumulative proof).
Requirements for admissibility of expert testimony under Rule 702 and the Daubert/Kumho line of cases.
Distinction between lay opinion (Rule 701) and expert opinion (Rule 702).
Proper subject matter for expert testimony: “scientific, technical, or other specialized knowledge” that helps the trier of fact.
Improper subjects for expert testimony, including legal conclusions, mental state in criminal cases, and credibility opinions.
Bases of expert opinion (Rules 703–705) and when supporting data may be revealed to the jury.
Use of Rule 403 and other rules to exclude expert testimony that is unreliable, unhelpful, or unduly prejudicial.

Test Your Knowledge

Attempt these questions before reading this article. If you find some difficult or cannot remember the answers, remember to look more closely at that area during your revision.

Which of the following is NOT a valid reason for excluding relevant evidence under Rule 403?
1. Unfair prejudice
2. Confusion of the issues
3. Lack of authentication
4. Undue delay
Under the Federal Rules, expert testimony is admissible only if:
1. The expert is a licensed professional in the field.
2. The testimony will help the jury understand the evidence or determine a fact in issue.
3. The expert has previously testified in court.
4. The expert’s opinion is based solely on personal experience.
Which standard governs the admissibility of scientific expert testimony in federal court?
1. Frye standard
2. Daubert standard
3. Best evidence rule
4. Confrontation Clause
A lay witness may give opinion testimony if:
1. The opinion is based on scientific, technical, or specialized knowledge.
2. The opinion is rationally based on the witness’s perception and helpful to understanding the testimony.
3. The witness is an expert in the field.
4. The opinion concerns the ultimate issue in the case.

Introduction

Relevant evidence is the backbone of any trial. But even when evidence has some probative value, it may still be excluded, especially when it comes to opinion testimony. Many of the hardest MBE questions in Evidence turn on:

Whether the testimony is lay or expert opinion.
Whether the topic is proper for expert testimony.
Whether the judge should exclude the evidence under Rule 403 or Rule 702.

Understanding these interactions is important for analyzing MBE fact patterns quickly and accurately.

Key Term: Relevant Evidence
Evidence that has any tendency to make a fact of consequence more or less probable than it would be without the evidence (Rule 401).

Relevancy: The Basic Standard

Relevance is a low bar:

The evidence need not prove the point by itself.
It only needs to “move the needle” on the probability of a fact of consequence in the case.

If evidence meets Rule 401’s definition of relevance, Rule 402 says it is admissible unless:

The U.S. Constitution, a federal statute, the Federal Rules of Evidence, or other rules require exclusion.
The judge exercises discretion under Rule 403 to exclude it.

In practice, the real fight is rarely over whether evidence is relevant at all, but whether it should nevertheless be kept out because of its dangers or because some specialized rule (like Rule 702) is not satisfied.

Reasons for Excluding Relevant Evidence

Even relevant evidence can be excluded when the cost outweighs the benefit.

Key Term: Rule 403 Exclusion
The court may exclude relevant evidence if its probative value is substantially outweighed by a danger of unfair prejudice, confusing the issues, misleading the jury, undue delay, wasting time, or needlessly presenting cumulative evidence.

Key points for Rule 403 on the MBE:

The balancing is strongly in favor of admissibility; the danger must substantially outweigh the probative value.
The prejudice must be unfair—evidence is almost always prejudicial in the ordinary sense.
The rule is discretionary; an MBE answer that treats 403 as mandatory in a close case is usually wrong.

Common 403 grounds:

Graphic photos in a relatively minor-injury case (unfair prejudice).
Complex expert testimony that will confuse rather than clarify (confusion/misleading).
Calling six identical experts to say the same thing (cumulative).

Importantly, the same Rule 403 standard applies to expert testimony as to any other type of evidence. Even if testimony satisfies Rule 702 and Daubert, the judge may still exclude it under Rule 403.

Proper Subject Matter for Expert Testimony

Opinion testimony falls into two broad categories:

Lay opinion (Rule 701).
Expert opinion (Rule 702).

The line between them is tested frequently.

Key Term: Expert Testimony
Testimony by a qualified person with scientific, technical, or other specialized knowledge whose opinion will help the trier of fact understand the evidence or determine a fact in issue (Rule 702).

Key Term: Scientific, Technical, or Specialized Knowledge
Knowledge beyond the ken of an average lay juror, gained through training, education, or specialized experience, rather than through ordinary life experience.

Under Rule 702, expert testimony is admissible only if:

The expert is qualified by knowledge, skill, experience, training, or education.
The testimony is based on sufficient facts or data.
The testimony is the product of reliable principles and methods.
The expert has reliably applied those principles and methods to the facts of the case.
The subject matter is scientific, technical, or specialized, and the testimony will help the trier of fact.

If these requirements are not met, the testimony is not merely weak—it is inadmissible.

Key Term: Rule 702
The rule that sets out the qualification, reliability, and helpfulness requirements for expert opinion testimony.

“Helpfulness” and the Proper Subject Matter

The “helpfulness” requirement is where the proper subject matter question lives. Expert testimony is proper only if:

The issue is beyond the understanding of an average juror; and
The expert’s specialized knowledge will assist the jury, not simply tell them what they can decide using common sense.

Examples of proper expert subject matter:

Medical diagnosis and causation of injury.
Accident reconstruction, ballistics, or engineering failures.
Economics: present value calculations, lost earning capacity.
Forensic analysis (DNA, fingerprints, blood spatter) when properly supported.

Examples of improper expert subject matter:

Purely legal questions (e.g., “this conduct was negligent,” “this contract is unconscionable”).
Credibility assessments (e.g., “the child is telling the truth,” “this pattern of behavior means she is lying”).
Common-sense issues the jury can evaluate without help (e.g., “the hallway was dark,” “the car was moving fast”).

Courts have generally rejected expert testimony on witness credibility. That includes “human lie detector” testimony and some forms of clinical profiling offered simply to say a witness is or is not truthful.

The Daubert Standard and Reliability

Key Term: Daubert Standard
The framework under which the judge, as gatekeeper, evaluates whether expert testimony is both reliable and relevant, using flexible factors such as testing, peer review, error rate, standards, and general acceptance.

Under Daubert v. Merrell Dow and Kumho Tire, the trial judge must act as a gatekeeper for all expert testimony—scientific, technical, or experiential. The judge has broad discretion to consider factors including:

Has the theory or technique been tested?
Has it been peer reviewed or published?
What is the known or potential error rate?
Are there standards controlling the technique’s operation?
Is it generally accepted in the relevant expert community?

No single factor is required; the inquiry is flexible and tailored to the field. But on the MBE, when you see a novel methodology with no testing, no peer review, unknown error rate, and no general acceptance, the right answer is almost always that the testimony is inadmissible under Rule 702.

Key Term: Gatekeeping Role
The trial judge’s duty under Rule 702 and Daubert to screen expert testimony for reliability and relevance before it reaches the jury.

Lay Opinion vs. Expert Opinion

Key Term: Lay Opinion
Opinion testimony by a non-expert, limited to perceptions based on personal knowledge, rationally based on the witness’s perception, helpful to understanding the testimony or deciding a fact in issue, and not based on specialized knowledge (Rule 701).

For lay witnesses (Rule 701), an opinion is admissible if:

It is rationally based on the witness’s own perception.
It is helpful to understanding the witness’s testimony or determining a fact in issue.
It is not based on scientific, technical, or specialized knowledge.

Permissible lay opinions commonly include:

Speed of a vehicle (“about 40 miles per hour”).
Appearance or condition (“she looked drunk,” “he seemed frightened,” “it was dark”).
Identification of handwriting or voice, if based on prior familiarity.
Approximate distance or weight.

Improper lay opinions include:

Medical diagnoses (“he had a concussion,” “she had a heart attack”).
Legal conclusions (“the driver was negligent,” “that was a binding contract”).
Opinions that depend on technical knowledge (e.g., calculations of blood alcohol content or “this fracture pattern shows intentional abuse”).

When a witness moves beyond common experience into specialized knowledge, the testimony must qualify as expert testimony under Rule 702, or it will be excluded.

Limits on Expert Testimony: Ultimate Issues and Mental State

The Federal Rules allow experts to be quite opinionated:

Key Term: Ultimate Issue
A fact that directly answers a key element in the case (e.g., whether a product was defective, whether a person was insane), as opposed to an intermediate fact from which the jury could infer the ultimate conclusion.

Under Rule 704(a), an opinion is not objectionable simply because it embraces an ultimate issue. For example:

A design engineer may opine that “the vehicle was defectively designed.”
A psychiatrist may opine that “the defendant had schizophrenia and could not appreciate the nature of his acts.”

However, there are two important limits:

Legal Conclusions – Even if phrased in ultimate-issue terms, experts cannot instruct the jury on what the law means or how it applies. Statements like “the defendant was negligent,” “the use of force was objectively unreasonable,” or “the contract was unconscionable” are usually excluded because they invade the court’s role.
Mental State in Criminal Cases – Rule 704(b) – In criminal cases, an expert may not state an opinion about whether the defendant did or did not have the mental state or condition constituting an element of the crime or a defense.

Key Term: Rule 704(b)
In a criminal case, an expert witness must not state an opinion about whether the defendant did or did not have a mental state or condition that constitutes an element of the crime or defense; that question is for the trier of fact.

Thus, a psychiatrist may describe diagnoses and symptoms, but cannot testify “the defendant lacked the intent to kill” or “the defendant knew his conduct was wrong.” Those ultimate mental state determinations belong exclusively to the jury.

Basis of Expert Opinions

Key Term: Basis of Expert Opinion
The set of facts or data, obtained through personal observation, evidence presented at trial, or information reasonably relied upon in the field, on which the expert builds an admissible opinion.

Under Rules 703–705, an expert may base an opinion on:

Personal observation (e.g., examining the patient, inspecting the accident scene).
Evidence presented at trial, including testimony and exhibits, often through hypothetical questions.
Information reasonably relied upon by experts in the field, even if those supporting facts would be inadmissible (e.g., hearsay in medical records, lab reports, data compilations).

Key exam points:

The expert’s opinion may be admitted even if the supporting information is inadmissible (such as hearsay), so long as experts in the field reasonably rely on that type of information.
The supporting inadmissible facts are not automatically disclosed to the jury. They may be revealed only if their probative value in evaluating the expert’s opinion substantially outweighs any prejudicial effect (a reverse 403 test).
On cross-examination, opposing counsel can inquire into the basis for the opinion and may bring out weaknesses in the facts or methodology.

Exclusion of Relevant Evidence: Rule 403

Rule 403 is the main mechanism for excluding relevant evidence. The court must weigh the probative value against the risk of unfair prejudice, confusion, misleading the jury, undue delay, wasting time, or needless cumulative evidence. The exclusion is discretionary and generally reserved for situations where the danger is significant and clearly outweighs the probative value.

Worked Example 1.1

A plaintiff sues for injuries after a car accident. The defendant wants to introduce evidence that the plaintiff was previously convicted of shoplifting. The plaintiff objects.

Answer:
The conviction has some probative value on the plaintiff’s credibility if the plaintiff testifies, but it says nothing about how the accident occurred. The risk is that the jury will dislike the plaintiff for being a shoplifter and may undervalue her injury claim. Under Rule 403, if the credibility issue is minor and the risk of unfair prejudice is substantial, the court may exclude the conviction. On these facts, exclusion is likely.

Proper Subject Matter for Expert Testimony (Rule 702)

Expert testimony is admissible only if:

The witness is qualified by knowledge, skill, experience, training, or education.
The testimony will help the jury understand the evidence or determine a fact in issue.
The testimony is based on sufficient facts or data.
The opinion is the product of reliable principles and methods.
The expert has reliably applied those methods to the facts.

When analyzing expert testimony for MBE purposes, walk through these steps:

Is the subject matter specialized (not lay)?
Is the expert qualified?
Is the methodology reliable under Daubert/Kumho?
Does the opinion logically fit the facts and help the jury?
Is there any Rule 403 reason to exclude?

Worked Example 1.2

In a products liability case, the plaintiff offers an engineer to testify that a machine was defectively designed. The expert’s opinion is based on published studies, industry standards, testing, and personal inspection of the machine.

Answer:
The engineer is qualified; design defect analysis is a technical subject beyond common understanding; the expert relied on recognized sources and applied standard engineering methods to the machine at issue. The testimony is likely reliable, relevant, and helpful, and should be admitted under Rule 702 and Daubert, subject to ordinary Rule 403 balancing.

Exam Warning

The court will exclude expert testimony that simply tells the jury what legal result to reach or states a legal conclusion (e.g., “the defendant was negligent,” “the force used was excessive under the Fourth Amendment”). Expert testimony must assist, not replace, the jury’s decision-making.

Limits on Lay Opinion

Lay opinion is limited to perceptions based on the witness’s own senses and must not involve scientific or technical knowledge. Lay opinion is allowed for things like speed, appearance, emotional state, and intoxication, but not for medical diagnoses, causation in complex accidents, or specialized technical matters.

Worked Example 1.3

A witness testifies, “In my opinion, the driver was drunk.” The witness observed the driver’s slurred speech, bloodshot eyes, and unsteady walk.

Answer:
This is permissible lay opinion. It is rationally based on the witness’s perception and helpful to the jury. The witness could not testify, “The driver’s blood alcohol level was above the legal limit,” because that requires scientific knowledge and testing; that would be expert subject matter.

Additional Examples of Proper vs. Improper Expert Topics

Proper: “In my opinion, given the patient’s symptoms and imaging, the fall caused a herniated disc.”
Improper: “The plaintiff is malingering and lying about his pain” (credibility).
Proper: “Based on regression analysis, the expected lost earnings are $500,000 in present value.”
Improper: “The defendant acted with reckless disregard for human life” (legal conclusion/mental state).

Worked Example 1.4

In a child sexual abuse case, the prosecution offers a psychologist to testify that “children in this age range rarely fabricate allegations of sexual abuse, and in my opinion, this child is telling the truth.”

Answer:
The testimony goes to credibility, not to a specialized issue beyond the jury’s understanding. Courts generally exclude “human lie detector” testimony—experts may not opine directly that a particular witness is truthful. The first part (generalization about children rarely fabricating) risks unfair prejudice and invades the jury’s role; the second part (this child is telling the truth) clearly invades the jury’s role. The testimony should be excluded.

Worked Example 1.5

A criminal defendant charged with intentional arson asserts an insanity defense. A psychiatrist is called by the defense to testify that the defendant has schizophrenia and, as a result, “could not appreciate the wrongfulness of his conduct and therefore lacked the required intent.”

Answer:
The psychiatrist may testify to diagnoses, symptoms, and general effects of schizophrenia—those are proper subjects of expert testimony. But under Rule 704(b), the expert may not state an opinion that the defendant lacked the required intent or “could not appreciate the wrongfulness of his conduct.” Those are ultimate mental state determinations reserved for the jury. The diagnosis and symptom testimony is admissible; the quoted mental state conclusion is not.

Worked Example 1.6

A structural engineer inspects a collapsed balcony and concludes that it failed because the support bolts were undersized according to building code. The plaintiff asks the expert, “Was the defendant negligent?” and the expert answers, “Yes, the defendant was negligent.”

Answer:
The engineer’s technical opinion about the cause of the collapse and deviation from code is proper expert subject matter. However, labeling the conduct as “negligent” is a legal conclusion and invades the court’s and jury’s roles. The court should allow testimony about the factual technical deficiencies (e.g., code violations, inadequate bolts), but exclude the bare legal conclusion “the defendant was negligent.”

Exclusion of Expert Testimony

Expert testimony may be excluded if:

The expert is not qualified in the relevant field.
The opinion is not based on sufficient facts or data.
The principles or methods are unreliable under Daubert.
The expert has not applied the methods reliably to the facts.
The subject matter is not helpful (within common sense or too speculative).
The opinion invades the jury’s role by stating legal conclusions, opining on mental state in a criminal case, or vouching for credibility.
The testimony’s probative value is substantially outweighed by unfair prejudice, confusion, or waste of time under Rule 403.

On the MBE, if you see a long, technical-sounding analysis with no testing, no peer review, unknown error rate, and no general acceptance, combined with an expert who wants to opine directly on the defendant’s guilt or mental state, the correct choice is almost always exclusion.

Key Point Checklist

This article has covered the following key knowledge points:

Relevant evidence under Rule 401 is admissible unless a specific rule or statute provides otherwise or the court excludes it under Rule 403.
Rule 403 permits exclusion of relevant evidence, including expert testimony, when its probative value is substantially outweighed by dangers such as unfair prejudice, confusion, or waste of time.
Lay opinion (Rule 701) must be rationally based on the witness’s perception, helpful, and not based on specialized knowledge.
Expert testimony (Rule 702) must involve scientific, technical, or specialized knowledge that will help the trier of fact, and must satisfy qualification, sufficiency of data, reliability of methods, and reliable application.
Under Daubert and Kumho, the judge acts as gatekeeper, using flexible reliability factors like testing, peer review, error rates, standards, and general acceptance.
Experts generally may express opinions on ultimate issues, but not legal conclusions and, in criminal cases, not the defendant’s mental state as an element of the crime or defense (Rule 704(b)).
Experts may rely on personal observation, evidence presented at trial, and information reasonably relied on by experts in the field, even if the latter is inadmissible, but supporting inadmissible facts are disclosed to the jury only under a strict balancing test.
Expert testimony on witness credibility or that simply states “the defendant is guilty” or “negligent” is improper because it invades the jury’s function.
Courts frequently use Rule 403 to exclude expert testimony that is only marginally helpful but highly confusing or prejudicial.

Key Terms and Concepts

Relevant Evidence
Rule 403 Exclusion
Expert Testimony
Daubert Standard
Lay Opinion
Scientific, Technical, or Specialized Knowledge
Ultimate Issue
Rule 702
Rule 704(b)
Basis of Expert Opinion
Gatekeeping Role

Relevancy and reasons for excluding relevant evidence - Prop...